In an imaging section of an image-forming apparatus such as a copying machine or a laser printer, a scanning system using a polygon mirror, or a LED head system performing high speed writing by using a head provided with a light emitting diode (LED) light source or an electro-luminescence (EL) light source is used. The latter system is preferred because it increases the copying speed and/or printing speed. However the latter system is inferior in an effective use of light because a LED light source or EL light source is generally a diffuse-light source. That is, in the imaging section generally consisting of a light source, an image-forming lens system, and a photosensitive drum, such a diffuse-light source is inferior in the amount of light to be applied to the image-forming lens system, thereby causing wasted power consumption. Furthermore, a diffuse-light source inferior in the amount of light to be applied to the image-forming lens system results ineffective use of light at the last photosensitive drum. For this reason, in order to increase the amount of light to be applied to the photosensitive drum, the effective area of the image-forming lens system (a self-focusing lens array which is a kind of rod lens array, called “SLA” hereinafter) is generally increased (that is, the number of SLAs is increased). However, since a SLA is costly, it is not economical to increase the number of SLAs.
On the other hand, there is a member which is not used for an image-forming apparatus such as a copying machine or printer, but is used to utilize diffuse light effectively. Such member is a lens array sheet for a liquid crystal display having optical elements which collimate the light supplied through a light transmission plate from the backlight device of the liquid crystal display toward the front side of a person viewing the liquid crystal display (see, for example, patent literatures 1 and 2).
When this member is used for an image-forming apparatus using a LED head system, since the light source is a diffuse-light source such as a LED light source or EL light source and has a distribution of light emission angles different from that of the light supplied through a light transmission plate from a backlight device, solid and minute optical elements tapered in an angle between 15° and 45° as specifically described in the above patent literature 1 can not apply light to SLAs effectively. For this reason, in case of a LED head system, a lens array sheet capable of applying the light from a LED light source or EL light source to SLAs more effectively, that is, in higher amount, is desired. Furthermore, the image-forming apparatus can be downsized by reducing the number of SLAs and the diameters of lenses constituting the SLAs.
On the other hand, a molded object such as a lens array sheet is required to have minute protrusions having a high aspect ratio, along with the requirement of miniaturizing a light source and/or increasing the amount of light to be utilized. It is becoming difficult to manufacture such a molded object with high accuracy by conventional technology.
As an existing method of manufacturing a precision plastic lens, there is what is called an injection molding method of injecting hot-melt resin into a metallic female mold under a certain temperature and pressure condition and cooling it (see, for example, patent literature 3). In addition, there is a photopolymer method of charging photosetting polymer, which is the material of a product, into a metallic mold, sealing it with material transparent to light for setting it, applying the light to the photosetting polymer through the transparent material to set it, and then releasing the product from the mold. At present, as a method of manufacturing a lens array with plastic, a plastic lens manufacturing method utilizing the above injection molding method or photopolymer method is used.
However, when optical elements constituting a lens array are minute structures which are arranged in high density and have a high aspect ratio, the above manufacturing method can not be used from resin-charging and product-releasing points of view. That is, when optical element lenses are not typical spherical lenses or non-spherical lenses, or when optical elements are shaped like a tapered corn and are minute structures which are sized in nanometers to microns and have a high aspect ratio, the viscosity of charged resin is still high even if the temperature of it is increased, and the resin and the metallic mold grapples together like a fastener after molded, and thereby it is difficult to release the whole of the molded resin from the mold. In order to release the molded resin from the mold easily, it has been tried that fluorinated resin or silicon resin is coated on the surface of the mold as release agent, but it has not been easy to release a lens array having protruded minute elements which are flexible, have a small taper angle and a high aspect ratio, and are arranged in high density.